Patent GB-1083954-A (GB)
Title: Control device
IPC: H01F19/06
Filed: 1966-01-28
Abstract: 1,083,954. Magnetic amplifiers; pilot operated A.V.C. AMPEX CORPORATION. Jan. 28, 1966 [Feb. 15, 1965], No. 3995/66. Headings H3B and H4R. [Also in Divisions G5 and H1] A control device comprises a differential transformer 34 in which the linkage between an output winding 53 and input windings 48, 49, 51 is varied by varying the reluctances of the yokes of the minimal hysteresis loop core 36 by control currents in bias and control windings 56, 58. If the device is to be used alternatively as an on/off switch or to provide linear variation of the coupling between primary and secondary windings but not for both together the bias windings 26 are omitted (Fig. 1, not shown). In the Fig. 2 arrangement the opposing primary fluxes in the core centre leg are in balance only when the control current is zero, the effect of the bias current being the same on both sides of the core. The control current aids the bias current on one side of the core and opposes it on the other thus producing unbalance in the opposing primary fluxes in the centre leg and an output in 53 which varies linearly with the control current. A small balance current may be provided from source 61 to adjust any asymmetry between the two sides of the core. In a slow switching system for play-back channels 1 and 2 of a magnetic video tape recorder first and second differential transformers of the kind shown in Fig. 2 having their primaries connected respectively to the channels 1 and 2 and their secondaries (53), (53<SP>1</SP>) (Fig. 3, not shown) in series to provide an output (at 64) are fed with control current of trapezoidal waveform from a generator and driver (72), (73). With bias currents of ¢ Ib from one bias source (69) and Ib from another bias source (68) and zero control current maximum unbalance and therefore maximum linkage is obtained in the second transformer between secondary (53<SP>1</SP>) and its associated primary windings and maximum output from the input at channel 2 (with zero output from channel 1 since the opposing primary fluxes in the limb of transformer (34) carrying secondary (53) are in balance). At the leading edge of the trapezoidal control current wave from the generator and driver (72), (73) the control current grows linearly reducing the unbalance in the second transformer (34) and producing increasing unbalance in the first transformer (34) until at the full amplitude of the control current wave maximum output appears (at 64) from channel 1 via the first transformer with zero output from channel 2. At the trailing edge of the control current wave the output switches back from channel 1 to channel 2. An automatic gain control system utilizing the differential transformer 34 of Fig. 2 is arranged to maintain an input signal, e.g. the playback signal from a video tape recorder, at a constant level. This is done by adding a pilot signal of predetermined frequency to the input signal and then separating the pilot signal from the rest of the output signal at 81, 82 (Fig. 4) and leading it, after amplification and demodulation to a differential amplifier 89 coupled to a D.C. reference source 90. The output is inversely related to the difference between the pilot and the reference and is applied as an error control current to the control windings 58 of the transformer 34. Increase of the pilot above a predetermined level causes decrease of the error control current with reduction of the coupling between input and output and consequent reduction of output and vice versa so that a substantially constant output is obtained. By splitting the two halves of the primary winding into two separate windings the differential transformer may be used as an analogue adder. It is also said to be usable as an analogue multiplier, AM modulator balanced modulator, single side band modulator, frequency synthesizer, frequency multiplexer, phase detector or timebase error detector.